Gas turbine engine

ABSTRACT

A bypass turbofan gas turbine engine is started by means of electric starter motor mounted directly about a downstream end or upstream of the low pressure spool of the engine. This causes air to be driven by a fan through a bypass duct around the engine casing. Closures close to substantially seal an outlet of the bypass duct, and the air is directed into a combustion chamber of the engine and through the turbines, causing the high pressure spool to pick up speed for starting.

This is a Division of application Ser. No. 11/898,591 filed Sep. 13,2007, which claims the benefit of British Patent Application No.0621074.4 filed Oct. 24, 2006. The disclosures of the prior applicationsare hereby incorporated by reference herein in their entirety.

The present invention relates to a gas turbine engine and particularlybut not exclusively to an apparatus and method for starting a bypassturbofan gas turbine engine.

In order to start a gas turbine engine, for example, a bypass turbofangas turbine engine, it is necessary to accelerate the high pressure (HP)spool to a speed high enough for sufficient air pressure and mass flowto be developed in the combustion chamber for fuel metered into thecombustion chamber to be ignited. After ignition of the fuel, fuel flowis increased until the engine reaches idle speed.

In one starting arrangement, pressurised air is impinged onto the HPturbine blades to impart sufficient momentum for the turbine to rotate.This arrangement requires pressurised air to be independently generated,for example, by means of a dedicated auxiliary air compressor.

In another starting arrangement, the HP spool driven by an electricstarter motor, which is positioned externally of the engine. The startermotor is connected to the HP spool through gears and a clutch mechanism.

The invention seeks to provide a starting arrangement for a gas turbineengine, which does not require the use of an external motor and gearingor an auxiliary compressor or other externally mounted starting device.

According to the present invention there is provided a gas turbineengine comprising an engine casing disposed around a low pressure spool,a high pressure spool and a combustion chamber; a bypass casing disposedaround the engine casing, a bypass duct disposed between the enginecasing and the bypass casing, a fan for supplying air to the bypass ductand a starter motor for rotating the fan on engine start-up,characterised in that the engine casing is provided with closableapertures which, when open, provide communication between a region ofthe bypass duct and the interior of the engine casing upstream of thecombustion chamber.

Preferably a first closure means is provided to reduce the flow area ofan outlet of the bypass duct or to substantially seal an outlet of thebypass duct between the bypass casing and the engine casing.

Preferably, a second closure means is disposed in the engine casing forallowing air to flow from the bypass duct through the aperture into thecombustion chamber when the second closure means is in an open positionand for sealing the aperture when the second closure means is in aclosed position.

Preferably, the first closure means is disposed downstream of the secondclosure means.

The second closure means may be biased to a closed position in which thebypass duct is sealed from the combustion chamber.

Preferably, the second closure means is positioned to allow airflowpassing through the bypass duct to flow into the engine at the upstreamend of the combustion chamber of the engine.

When the electric starter motor rotates the LP spool, the airflowgenerated by a LP fan at the upstream end of the engine takes the pathof least resistance, ie passes through the bypass duct. The airflowthrough the engine is therefore minimal. In order to maximise this coreairflow, the first closure means seals the outlet of the bypass duct andthe second closure means opens the bypass duct to the combustion chamberand turbines of the engine.

In one aspect of the invention the first closure means is configurableto allow airflow from the bypass duct through the aperture into thecombustion chamber when the closure means is in a first position forstarting; and to allow airflow through the outlet of the bypass duct andto seal the combustion chamber from the bypass duct when the closuremeans is in a second position for engine operation.

The closure means may have a single actuated member.

In all embodiments of the invention, the starter motor may operate as agenerator when the engine is operating.

The engine may be a multi-spool bypass turbofan engine.

According to a further aspect of the present invention there is provideda method of starting a gas turbine engine comprising an engine casingdisposed around at least one low pressure spool, a high pressure spooland a combustion chamber (24); a bypass casing (30) disposed around theengine casing (28), a bypass duct (32) disposed between the enginecasing (28) and the bypass casing (30), characterised by directingairflow from the bypass duct (32) into the upstream end of a combustionchamber (24) of the engine (10) through at least one closeable aperture(31) in the engine casing (28).

Preferably, the method further comprises reducing the flow area of anoutlet of the bypass duct or substantially sealing an outlet of thebypass duct, and allowing airflow passing through the bypass duct to bedirected into the combustion chamber.

Ideally, in all embodiments of the invention, airflow can be directedinitially into the engine onto the turbine blades without passingthrough the HP compressor.

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings, in which:-

FIG. 1 shows a schematic cross-sectional view through a multi-spool gasturbine engine in accordance with the invention in an initial stage in astarting procedure;

FIG. 2 shows a schematic cross-sectional view through the gas turbineengine of FIG. 1 in a second stage in the starting procedure;

FIG. 3 shows a schematic cross-sectional view through the gas turbineengine of FIG. 1 in a third stage in the starting procedure;

FIG. 4 shows a schematic cross-sectional view through an alternativeembodiment of multi-spool gas turbine in an operating condition inaccordance with the invention; and

FIG. 5 shows a schematic cross-sectional view through the engine of FIG.4 in a starting condition.

Referring firstly to FIG. 1, a first embodiment of multi-spool gasturbine engine is indicated generally at 10. The engine 10 isconventional in that it comprises three spools, that is to say itincludes a low pressure (LP) spool 12, an intermediate pressure (IP)spool 13, and a high pressure (HP) spool 14. However the invention mayequally be applied to any single spool, twin spool or multi spool enginearrangement. A fan 16 is mounted on the front or upstream end of the LPspool, IP booster stage blades 18 are mounted on the IP spool 13, and acompressor 20 is mounted on the HP spool 14. HP turbine blades 22 areconnected to the compressor 20, ie on the HP spool, IP turbine blades 21are mounted on the IP spool 13 and LP turbine blades 23 are mounted onthe LP spool. A combustion chamber 24 is downstream of the compressor20, but upstream of the turbine blades 22, 21, 23.

An electrically driven starter motor 26 is mounted directly about thedownstream end of the LP spool 12, which may be axially extended forthis purpose, within the nozzle of the engine. The starter motor 26 mayalternatively be mounted directly about the upstream end of the LP spool12, within the nose cone of the engine. An engine casing 28 surroundsthe IP booster stage blades 18, the HP compressor 20, the combustionchamber 24 and the turbine blades 22, 21, 23. A bypass casing 30surrounds the fan 16 and extends around and along the engine casing 28,creating a substantially annular bypass duct 32 between the enginecasing 28 and the bypass casing 30.

A plurality of bypass duct closures 34, two of which are shown, areprovided in the bypass casing 30 equi-spaced around the casing 30 at thedownstream end of the engine, proximate the outlet of the bypass duct32. The closures 34 are movable to seal the outlet of the bypass ductbetween the bypass casing 30 and the engine casing 28. The closures 34comprise a plurality of actuated flaps. The seal made may be a completeseal (that is to say does not permit any leakage to the outlet of thebypass duct 32) or a partial seal (that is to say, it permits acontrolled leakage to the outlet of the bypass duct 32). Alternativelythe closures 34 may be moveable to reduce the flow area of the bypassduct 32.

A plurality of apertures 31 are disposed in the engine casing 28 at theupstream end of the combustion chamber 24. Engine casing closures 36 aremounted inside the engine casing 28 to cover and seal the apertures 31.When open, as shown in FIG. 2, the closures 36 allow air to pass throughapertures 31 from the bypass duct 32 into the combustion chamber 24. Theengine casing closures 36 are spring loaded to a closed position asindicated in FIG. 1, which shows the engine in an operating condition.Although two apertures 31 and engine casing closures 36 are shown, aplurality are provided, equi-spaced around the engine casing 28.

In order to start the engine 10, initially the bypass duct closures 34and engine casing closures 36 are in the positions shown in FIG. 1, iewith the engine casing closures 36 closed and the bypass duct closures34 open. The starter motor 26 operates to drive the LP spool, causingthe fan 16 to push air through the engine. When the LP spool reaches asufficient speed, the bypass duct closures 34 are moved to the positionas shown in FIG. 2 such that they close, or partially close, the outletfrom the bypass duct 32. As a result, air pressure in the bypass duct 32between the fan and the engine casing closures 36 increases, causing theengine casing closures 36 to be opened against the spring force,allowing air to be directed from the bypass duct 32 through theapertures 31 into the upstream end of the combustion chamber 24 andthrough the turbines 22, 21, 23. The majority of air reaching thecombustion chamber 24 has bypassed the high pressure compressor 20 byflowing through the bypass duct 32 and the apertures 31. The airtravelling through the combustion chamber 24 impinges on the HP turbinewith enough momentum to begin initial rotation of the HP spool.

The HP spool 14 begins to accelerate and draws more air in through themouth of the engine core. This drawing in of more air results incontinued acceleration of the HP spool 14 and raises the pressure in thecombustion chamber 24. This pressure rise continues until the pressurereaches a level where it equals that of the air pressure in the bypassduct 32. By this point, the pressure difference across the engine casingclosures 36 will have reduced to the extent that the engine casingclosures close under the action of the springs against the engine casing28. This produces the required seal to accommodate the continuedpressure rise associated with the increasing speed of the HP spool 14.The bypass duct closures 34 are configured to prevent the build up ofpressure in the bypass duct 32. That is to say they may remain closed,as shown in FIG. 3, or may be partially open, or may oscillate betweenbeing fully open, partially open and/or closed as required to prevent apressure build up which results in aerodynamic instabilities that affectthe operation of the fan 16.

If the bypass duct closures 34 were now fully opened, the high pressurespool 14 may begin to slow down. Therefore, the bypass duct closures 34remain closed, or at least partially closed, until successful ignitionof fuel air mixture in the combustion chamber 24 can be achieved, oreven longer to ensure that idle speed can be reached in the same way aswith a conventional starting arrangement after ignition. Thereafter, thebypass duct closures 34 are opened and the engine 10 operates in thecondition shown in FIG. 1.

In an alternative embodiment to that described above in relation toFIGS. 1 to 3, the closures 34,36 are activated and controlledhydraulically, pneumatically, electrically or by some other suchsuitable method.

Referring now to FIGS. 4 and 5, a second embodiment of a multi spool gasturbine engine is indicated at 40. Common references numerals have beenused to designate parts in common with the first embodiment described.In this embodiment, the bypass duct closures 34 and engine casingclosures 36 of the previous embodiment described are integrated intoactuated closures 42, each comprising a single flap 44. A plurality ofthese flaps 44 are hinged to the engine casing 28 and are disposedequi-spaced about the bypass duct 32. When activated, for example eitherhydraulically, pneumatically or electrically, the flap 44 of eachclosure 42 moves between a normal operating position, in which it coversthe aperture 31 in the engine casing 28 near the combustion chamber 28and leaves the bypass duct 32 unobstructed, and a starting position(FIG. 5), in which the aperture 31 is exposed allowing air flow into thecombustion chamber 24 and substantially closing the bypass duct 32 at aposition downstream of the aperture 31.

The engine 40 is started in the same way as the engine 10, save thatonce the LP spool 12 has reached a sufficient speed with the closures 42in the starting position, the closures are closed for ignition, ie thecombustion chamber 24 is sealed from the bypass duct 32.

The invention is intended to include any physical arrangement forsubstantially sealing the outlet of the bypass duct 32 and allowing airflow into the combustion chamber 24 and through the turbine blades 22,23 of the engine. When operating, the starter motor 26 functions as agenerator.

In use on a jet aircraft, the invention also gives the advantage ofproviding means for varying core and bypass mixing areas by operation ofthe closures for optimising mixing throughout the flight envelope.

1. A method of starting a gas turbine engine comprising an engine casingdisposed around a compressor, and a combustion chamber; a bypass casingdisposed around the engine casing, a bypass duct disposed between theengine casing and the bypass casing, characterised by directing airflowfrom the bypass duct into the upstream end of a combustion chamber ofthe engine through at least one closeable aperture in the engine casing.2. A method of starting a gas turbine engine as claimed in claim 1characterised by reducing the flow area of an outlet of the bypass ductor by substantially sealing an outlet of the bypass duct and allowingairflow passing through the bypass duct to be directed into thecombustion chamber.